1. Field of the Invention
The present invention relates to projection exposure apparatus method, used in a lithographic process in the manufacture of semiconductor integrated circuits and liquid crystal devices and, more particularly, to maintenance and adjustment of imaging performance of a projection optical system.
2. Related Background Art
In a photolithographic process for forming circuit patterns as of semiconductor elements, there is employed a method of transferring a pattern formed on a reticle (mask) onto a substrate (e.g., a semiconductor wafer or glass plate). A photoresist having photosensitive properties is applied to the substrate, and a circuit pattern is transferred to the photoresist in accordance with an irradiating optical image, i.e., a pattern shape of a transparent portion of a reticle pattern. In a projection exposure apparatus (e.g., a stepper), a reticle pattern image is imaged and projected on the wafer through a projection optical system.
In an apparatus of this type, illumination light is limited to have an almost circular (rectangular) shape centered on the optical axis of the illumination optical system to illuminate the reticle on an illumination optical system plane (to be referred to as a pupil plane of an illumination optical system hereinafter) serving as a Fourier transform plane for a surface on which a reticle pattern is present or a plane near the pupil plane of the illumination optical system (this illumination scheme is called normal illumination hereinafter). For this reason, the illumination light is incident on the reticle at almost a right angle. A circuit pattern having a transparent portion (i.e., the naked surface of the substrate) having a transmittance of 100% for the illumination light and a light-shielding portion (chromium or the like) having a transmittance of almost 0% is formed on a reticle (i.e., a glass substrate as of quartz) used in this apparatus.
The illumination light radiated on the reticle, as described above, is diffracted by the reticle pattern, and 0-th-order and .+-.1st-order diffracted light components emerge from the pattern. These diffracted light components are focused by the projection optical system to form interference fringes, i.e., reticle pattern images on the wafer. An angle defined by the 0-th-order diffracted light component and each of the .+-.1st-order diffracted light components is defined as sin.theta.=.lambda./P where .lambda. is the wavelength (.mu.m) of the exposure light, and NA is the numerical aperture of the projection optical system on the reticle side.
When a pattern pitch is decreased, the sin.theta. value is increased. When the sin.theta. value is larger than the numerical aperture NA of the projection optical system on the reticle side, the .+-.1st-order diffracted light components are limited by the effective diameter of the projection optical system plane (to be referred to as a pupil plane of the projection optical system hereinafter) serving as the Fourier transform plane of the reticle pattern and cannot pass through the projection optical system. That is, only the 0-th-order diffracted light component can reach the wafer, and the interference fringes (pattern image) are not formed. In the conventional exposure method described above, when the reticle having only the transparent and light-shielding portions (to be referred to as a normal reticle hereinafter) is used, the degree of micropatterning of the reticle pattern (minimum pattern pitch) P which can be resolved on the wafer is given as P.congruent..lambda./NA since sin.theta. =NA. The minimum pattern size is 1/2 the pitch P, and the minimum pattern size is given as about 0.5.times..lambda./NA. In a practical photolithographic process, however, a given depth of focus is required due to warping of the wafer, influences of steps on the wafer during the process, and the thickness of the photoresist itself. For these reasons, the practical minimum resolution pattern size is represented as k.times..lambda./NA where k is the process coefficient which generally falls within the range of about 0.6 to 0.8.
In order to expose and transfer a fine pattern in accordance with the conventional exposure method, an exposure light source which emits light having a shorter wavelength or a projection optical system having a larger numerical aperture must be used.
However, it is difficult to arrange an exposure light source which emits light having a shorter wavelength (e.g., 200 nm or less) than that of the existing exposure light source at present because an optical material suitably used as a light-transmitting optical member is not available and a stable light source capable of emitting a large amount of light is not available either. In addition, the numerical aperture of the state-of-the-art projection optical system is almost a theoretical limit, and the numerical aperture is assumed not to drastically increase. Even if the numerical aperture can be larger than that currently used, the depth of focus determined by .+-..lambda./NA.sup.2 abruptly decreases with an increase in numerical aperture, and the depth of focus used in practice further decreases, resulting in inconvenience.
There is also proposed a phase shifting reticle having a phase shifter (e.g., a dielectric thin film) for shifting light transmitted through a specific one of transparent portions of the reticle circuit patterns from light transmitted through another transparent portion by .pi. (rad). A phase shifting reticle is disclosed in Japanese Patent Publication No. 62-50811. When this phase shifting reticle is used, a finer pattern can be transferred as compared with use of the normal reticle. That is, a resolving power can be increased. In order to use this phase shifting reticle, the numerical aperture (coherence factor .sigma.) of the illumination optical system must be optimized. Various schemes are proposed for the phase shifting reticle, and typical examples are a spatial frequency modulation scheme, a shifter light-shielding scheme, and an edge emphasis scheme.
In recent years, various attempts have been made to allow transfer of micropatterns in accordance with optimization of illumination conditions (LIT CONDI) or implementations of an exposure method. As described in U.S. Pat. No. 4,931,830, there is provided a method of increasing the resolving power and the depth of focus for patterns having specific line widths, in such a manner that a combination of an optimal numerical aperture (value .sigma.) of the illumination optical system and an optimal numerical aperture (N.A.) of the projection optical system is selected every pattern line width. In addition, an annular illumination method is also proposed in which a light amount distribution of illumination light on or near the pupil plane of the illumination optical system is defined to have an annular shape, and a reticle pattern is irradiated with the annular illumination light. There is also proposed an oblique illumination method in which the light amount distribution of the illumination light on or near the pupil plane of the illumination optical system is set to have maximal values at a plurality of positions eccentric from the optical axis of the illumination optical system, and illumination light is inclined at a predetermined angle in correspondence with the periodicity of a reticle pattern and is incident on the reticle pattern from a specific direction, as described in PCT/JP91/01103 (Aug. 19, 1991) and Ser. No. 791,138 (Nov. 13, 1991). Any one of the methods described above is not effective for all reticle patterns, i.e., line widths and shapes of the patterns. Optimal illumination method and conditions must be selected every reticle or patterns thereof. The projection exposure apparatus must have a structure in which illumination conditions (e.g., the value a) in the illumination optical system must be set variable.
In the manufacture of semiconductor integrated circuits, although a high resolving power is required, a large depth of focus (focus margin) also serves as an important factor.
A method of increasing the depth of focus by giving a specific aberration (particularly, a spherical aberration) to a projection optical system without using the modified light source and the like described above is proposed in Japanese Laid-Open Patent Application No. 2-234411. This method utilizes the thickness (about 1 .mu.m) of a photosensitive material (photoresist) on the wafer. The depth of focus can be increased although the contrast level is slightly decreased.
In a recent projection exposure apparatus, imaging characteristics of the projection optical system are required to be constant with high precision. Various methods of adjusting the imaging characteristics are proposed and put into practice. Among them all, a method of correcting variations in imaging characteristics which are caused by exposure light absorption in the projection optical system is disclosed in U.S. Pat. No. 4,666,273. In this method, an energy amount (heat) accumulated in the projection optical system upon incidence of exposure light on the projection optical system is sequentially calculated, an amount of change in each imaging characteristic caused by the accumulated energy amount is obtained, and each imaging characteristic is finely adjusted by a predetermined correction mechanism. This correction mechanism can be arranged in accordance with, e.g., a scheme for sealing a space defined by two of a plurality of lens elements constituting the projection optical system and adjusting the pressure of the sealed space.
The imaging characteristics corrected as described above are a projection magnification, the focal position, the distortions, and the like. In a projection optical apparatus which requires high-precision control of imaging characteristics, other imaging characteristics such as the curvature of field, which characteristics are not conventionally corrected because the correction is difficult and variations in accumulated energy amount (thermal accumulation amount) are small, are also taken into consideration as correction targets. There is proposed a method in which the limit value (reference value) of the thermal accumulation amount is determined in advance so that variations in imaging characteristics (caused by exposure light absorption) corresponding to the thermal accumulation amounts of the projection optical system do not exceed predetermined amounts. According to this method, in sequential exposure of wafer with a reticle pattern in accordance with a step-and-repeat scheme, when an actual thermal accumulation amount of the projection optical system exceeds the reference value, the exposure operation is stopped, and the exposure operation is inhibited (kept stopped) until the actual thermal accumulation amount becomes below the reference value. More specifically, in the same manner as in the technique disclosed in the prior art described above, the thermal accumulation amount of energy accumulated in a projection optical system upon incidence of exposure light thereon is sequentially calculated, and the calculated thermal accumulation amount is compared with a predetermined reference value every shot in exposure or every exchange of the wafer to determine whether the exposure operation for the next shot is performed.
Since the method disclosed in U.S. Pat. No. 4,666,273 does not directly detect the imaging characteristics of the projection optical system, it is necessary to directly detect the imaging characteristics of the projection optical system by using a focal position detection system, the patent application of which is filed as Ser. No. 830,213 (Jan. 30, 1992). This focal position detection system will be described with reference to FIGS. 30 to 33.
Referring to FIG. 30, a reticle R on which circuit patterns are drawn is held in a reticle holder 14. The reticle R is illuminated at a uniform illuminance with exposure light such as bright lines from a mercury lamp or an excimer laser beam. A wafer W is held on a wafer holder 16 placed on a wafer stage WS. In a normal exposure transfer mode, the pattern on the reticle R is imaged on the wafer W through a projection optical system PL.
The wafer stage WS is constituted by an XY stage movable within a plane (to be referred to as an XY plane hereinafter) perpendicular to the optical axis of the projection optical system PL and a Z stage movable above the XY stage in the Z direction parallel to the optical axis of the projection optical system PL. The wafer holder 16 also serves as a .theta. stage which can be slightly rotated within the XY plane. Coordinates of each exposure point corresponding to the optical axis of the projection optical system PL are measured by a biaxial laser interferometer (not shown). Z coordinates of the wafer W on the exposure surface upon movement of the Z stage in the Z direction can also be measured by a measuring mechanism (not shown). When the wafer stage WS is moved within the plane perpendicular to the optical axis of the projection optical system PL, the pattern on the reticle R is exposed on the wafer W in accordance with the step-and-repeat scheme. When the wafer stage WS is also slightly moved in the axial direction of the projection optical system PL, the wafer W can be matched with the focal position of the projection optical system PL.
A focal position detection pattern plate (reference member) 15 is disposed on the wafer stage WS. An aperture pattern (multislit pattern) 15A constituted by light-shielding and transparent portions is formed on the upper surface of the pattern plate 15, as shown in FIG. 31A. This aperture pattern 15A is constituted such that four amplification type diffraction gratings each consisting of lines and spaces at a predetermined pitch are rotated through every 90.degree.. The pattern plate 15 is fixed on the wafer stage WS such that the surface (i.e., the surface on which the aperture pattern 15A is formed) of the wafer stage WS is located at almost the same level as that of the exposure surface (upper surface) of the wafer W with respect to the Z direction (i.e., the axial direction of the projection optical system PL). A detection illumination optical system is arranged below the lower surface (i.e., inside the wafer stage WS) of the pattern plate 15.
In the detection illumination optical system, illumination light EL having a wavelength range which is the same as or close to that of exposure light for illuminating the reticle R is incident on one branch end 110a of a two-split fiber bundle 110. The illumination light EL is light obtained by splitting part of exposure light IL by means of a beam splitter or the like. The illumination light EL is guided from the branch end 110a to a merged end 110c and is supplied to the inside of the wafer stage WS. The illumination light EL illuminates the aperture pattern 15A of the pattern plate 15 upward through an output lens 111, a field stop 113, a relay lens 114, a mirror 115, and a condenser lens 116. The beam passing through the pattern plate 15 forms an image of the aperture pattern 15A of the pattern plate 15 on the lower surface (pattern surface) of the reticle R through the projection optical system PL. Light reflected by the pattern surface of the reticle R returns again to the inside of the wafer stage WS through the projection optical system PL and the pattern plate 15. The light is then incident on the merged end 110c of the fiber bundle 110 along a path opposite to the incident path. This reflected light emerges from the other branch lens 110b of the fiber bundle 110 and is incident on a photoelectric sensor PD. The photoelectric sensor PD outputs a focus signal FS, i.e., a photoelectric signal corresponding to the amount of beam reflected by the pattern surface of the reticle R and passing through the aperture pattern 15A of the pattern plate 15, to an autofocus controller (AFC) 35.
According to this scheme, when the focus signal FS output from the photoelectric sensor PD has a maximum magnitude, i.e., when the amount of light obtained by limiting the light reflected by the reticle R by means of the pattern plate 15, the corresponding Z coordinates are detected as a focal position. The principle of the maximum amount of light at the focal position will be described below with reference to FIGS. 32A to 32C. FIG. 32A shows an optical path diagram in which the aperture pattern formation surface of the pattern plate 15 is conjugate (imaging relationship) with the pattern surface of the reticle R with respect to the projection optical system PL, i.e., the upper surface of the pattern plate 15 is located at the focal position of the projection optical system PL. The beam passing through the transparent portion of the pattern plate 15 toward the projection optical system PL forms the focused image of the aperture pattern 15A on the lower pattern surface of the reticle R, and its reflected light also forms a focused image on the pattern plate 15. For this reason, the aperture pattern 15A of the pattern plate 15 and the refocused aperture pattern image are perfectly superposed on each other. The image of the aperture pattern (i.e., the imaging beam) directly passes through the pattern plate 15 and is finally incident on the photoelectric sensor PD.
On the other hand, FIG. 32B shows an optical path diagram in which the aperture pattern formation surface of the pattern plate 15 is not located at the focal position of the projection optical system PL. In this case, the light reflected by the lower surface of the reticle R cannot be entirely transmitted through the aperture pattern 15A of the pattern plate 15, so that part of the reflected light is reflected by the light-shielding portion (hatched portion) of the aperture pattern 15A. The amount of light incident on the photoelectric sensor PD is reduced. In practice, since an interference phenomenon between the rays occurs, the focus signal FS corresponding to the amount of light obtained by limiting the light reflected by the reticle R by means of the pattern plate 15 has a waveform shown in FIG. 31B. Referring to FIG. 31B, an autofocus signal AFS detected by an indirect focus position detection system (30 and 31) (to be described later) is plotted along the abscissa. This signal AFS corresponds to the Z coordinates of the wafer stage WS.
Referring to FIG. 30, a beam emitted from a light-emitting optical system 30 is incident obliquely on the pattern plate 15 with respect to the optical axis of the projection optical system PL. For example, a slit-like pattern is projected on the pattern plate 15. Light reflected by this pattern plate 15 is projected on the light-receiving element of a light-receiving optical system 31, and an image of the slit pattern formed on the pattern plate 15 is refocused on the light-receiving element of the light-receiving optical system 31. When the pattern plate 15 is moved in the Z direction parallel to the optical axis of the projection optical system PL, the slit pattern image on the light-receiving element of the light-receiving optical system 31 is also moved. The position of the aperture pattern formation surface (or the exposure surface of the wafer W) in the Z direction can be detected from the slit pattern position.
The light-receiving optical system 31 outputs the signal (to be referred to as an autofocus signal) AFS corresponding to the position of the slit pattern image. The autofocus signal AFS is supplied to the autofocus controller 35. The autofocus controller 35 also receives the focus signal FS of the direct scheme from the photoelectric sensor PD. The autofocus controller 35 performs offset adjustment of the autofocus signal AFS by using the focus signal FS of the direct scheme. A Z-axis drive signal ZS is supplied to a motor 17 so that the autofocus signal AFS is set at a predetermined level, thereby driving the Z stage of the wafer stage WS. A technique for performing calibration (offset adjustment) of a focusing mechanism of an indirect scheme using the detection result of the direct scheme is disclosed in U.S. Pat. No. 4,650,983.
A method of focusing the exposure surface of the wafer W with respect to the projection optical system PL by using the focal position detection system having the pattern plate 15 will be briefly described below. As described above, the distance between the wafer W and the projection optical system PL is adjusted by a wafer position detection system comprising the light-emitting optical system 30 and the light-receiving optical system 31 during exposure of the wafer W. For this reason, the focal position obtained in the focal position detection system including the pattern plate 15 upon movement of the pattern plate 15 to the central portion of the image field of the projection optical system PL is fed back to the wafer position detection system comprising the light-emitting optical system 30 and light-receiving optical system 31. Even if variations in focal position of the projection optical system PL may be caused by variations over time, the exposure surface of the wafer W can always be focused at the varied focal position. This feedback operation, i.e., the calibration operation of the wafer position detection system comprising the light-emitting optical system 30 and the light-receiving optical system 31 is performed every unit time, every wafer, or every several wafers.
The autofocus controller 35 for controlling the focusing operation moves the wafer stage WS along the direction of the optical axis of the projection optical system PL in accordance with the Z-axis drive signal ZS. At the same time, the autofocus controller 35 receives the autofocus signal AFS representing the position of the pattern plate 15 from the light-receiving optical system 31 and the focus signal FS from the photoelectric sensor PD. As a result, the waveform shown in FIG. 31B can be obtained. A value BS of the autofocus signal AFS at the peak position of the waveform represents the focal position of the projection optical system PL. Thereafter, the wafer W is focused so that the value of the autofocus signal AFS as an output from the light-receiving optical system 31 becomes the value BS. As described above, without arranging a special pattern in the reticle R, focal position detection of an arbitrary point and particularly the central point within the image field of the projection optical system PL can be accurately performed according to this technique. According to this conventional technique, however, the aberration (particularly, the spherical aberration) of the projection optical system is fixed during the manufacture or adjustment and cannot be easily changed later.
When the annular illumination, the modified light source, and the phase shifting reticle are applied to a projection optical system having a predetermined spherical aberration for normal illumination so as to increase the depth of focus, the predetermined spherical aberration adversely affects the projection exposure. More specifically, the depth of focus is reduced in accordance with the spherical aberration, and a change in focal position occurs due to a line width (pitch) in use of the annular illumination, the modified light source, and the phase shifting reticle. For this reason, the effective depth of focus is reduced as compared with the case wherein the predetermined spherical aberration is not provided.
Even if an optical system does not have the spherical aberration, distortion or the like may occur due to a change in illumination condition.
The position and shape of a beam within the pupil plane of the projection optical system through which the beam passes are variously changed in accordance with a change in illumination light shape and use of a phase shifting mask. If the numerical aperture of the projection optical system is defined as NA and the numerical aperture of the illumination system is defined as NAi, a .sigma. value as one of the parameters of the illumination system is defined as .sigma.=NAi/NA. This .sigma. value is conventionally about 0.5. When a conventional illumination method (normal illumination) or a normal reticle is used, light components diffracted from the reticle are spread on the entire aperture (pupil plane) including the optical axis of the projection optical system within the pupil plane. To the contrary, the .sigma. value is set to be, e.g., about 0.3 in the phase shifting reticle.
Since the beam corresponding to the optical axis is lost within the pupil plane in accordance with the principle of phase shifting, a beam passes through only part of the peripheral portion within the pupil plane of the projection optical system. In addition, even if annular illumination or oblique illumination is performed, a beam corresponding to the optical axis is lost within the pupil plane in the same manner as in the phase shifting mask, and a beam passes through only the region of the peripheral portion within the pupil plane.
As described above, when the .sigma. value is changed or the state of the beam passing within the pupil plane is changed, a slight variation occurs in the focal position of the projection optical system by the small spherical aberration remaining in the projection optical system and the slight light absorption of the projection optical system.
When the illumination condition of the illumination optical system is changed or the phase shifting reticle is used as described above, the distribution of an amount of light transmitting through the interior of the projection optical system, and particularly a portion near the pupil (of the lens element) is changed. This change in light amount distribution greatly adversely affects variations in imaging characteristics caused by illumination light absorption of the projection optical system. That is, the change characteristics (e.g., a rate of change and a time constant) of the imaging characteristics are also changed. More specifically, even if the total sum (i.e., the thermal accumulation amount) of the energy amounts of the illumination light incident on the projection optical system is kept unchanged, the change characteristics of the imaging characteristics change depending on different illumination conditions. Even if a given reference value is used, aberration amounts may be different. For this reason, when exposure is to be performed by changing the illumination condition described above, the reference value for executing the exposure operation and inhibiting it is not a constant value, and the conventional method cannot be directly applied. The imaging characteristics are degraded depending on the illumination condition. The exposure operation is inhibited even if a detected aberration amount does not exceed the predetermined amount, thereby reducing productivity (throughput), resulting in inconvenience.
As disclosed in Ser. No. 464,621 (Jan. 3, 1990), calculation parameters used for calculating change amounts of the imaging characteristics of the projection optical system upon changes caused by illumination light absorption may be corrected for each illumination condition, and the changes in imaging characteristics caused by the change in illumination condition are accurately obtained to perform correction by using the corrected parameters.
There are, however, a very large number of combinations of arrangements of illumination optical systems, illumination conditions (i.e., a .sigma. value, annular illumination, and a modified light source), types of reticles, and numerical apertures of projection optical systems. For this reason, preparation of operation parameters for all the combinations is very complicated and requires a long period of time. In particular, as for the types of reticles, when a reticle pattern such as a degree of micropatterning (e.g., a pitch and a line width) and a periodic direction slightly changes, emergence of diffracted light from the pattern varies. For this reason, it is very difficult (practically impossible) to prepare operation parameters corresponding to the illumination conditions for all the types of reticles.
When the imaging characteristics are corrected as described above, no problem is posed on a long-term basis. However, the thermal accumulation phenomenon of the projection optical system has a past history or hysteresis. When the illumination condition is changed in correspondence with the reticle and its pattern, and calculation and correction of amounts of changes in imaging characteristics are immediately started using the corrected calculation parameters under the updated illumination condition, the imaging characteristics cannot be properly corrected while the history corresponding to the previous illumination condition is left in the projection optical system. More specifically, upon the change in illumination condition, a lens element near the pupil plane of the projection optical system is set in a state wherein the thermal distribution state of the previous illumination condition is mixed with that of the updated illumination condition, and the current state cannot be specified as one of the above states. Therefore, even if the amounts of changes in imaging characteristics are calculated in accordance with the calculation parameters under either illumination condition, the calculation result does not match with the actual amounts of changes in imaging characteristics. The imaging characteristics in this transient state (i.e., the thermal profile state of the projection optical system) cannot be simply expressed as the sum of these states. It is therefore very difficult to accurately calculate and correct the amounts of changes in imaging characteristics in the transient state. Therefore, even if pattern exposure on the wafer in this transient state is performed, a circuit pattern which satisfies the prescribed characteristics cannot be obtained.
In the apparatus shown in FIG. 30, since a new illumination system which is not an exposure illumination system is used to perform focal position detection using the pattern plate 15, the characteristics of these illumination systems do not coincide with each other, strictly speaking. When the characteristics of these two illumination systems do not coincide with each other, the focal position of the projection optical system PL, which is detected using the exposure light IL, may be offset from the focal position of the projection optical system PL, which is detected using the illumination light EL.
Values representing the characteristics of the illumination system are generally the numerical aperture NA of the projection optical system and the a value representing the coherency of the illumination light. The numerical aperture and the .sigma. value will be described with reference to FIG. 33. Referring to FIG. 33, since an aperture stop 32 is arranged at a pupil plane Ep of the projection optical system PL, i.e., the Fourier transform plane, a maximum angle .theta..sub.R at which a beam can pass through the projection optical system PL from the reticle R and a maximum angle .theta..sub.W of a beam incident from the projection optical system PL to the pattern plate 15 are limited to predetermined values. The numerical aperture NA.sub.PL of the projection optical system PL is sin.theta..sub.W. When a projection magnification is defined as 1/m, equation Sin.theta..sub.R =sin.theta..sub.W /m is established.
A value .sigma..sub.IL as the .sigma. value on the exposure light IL side and .sigma..sub.EL as the .sigma. value on the illumination exposure light EL side are defined as follows: ##EQU1## where .theta..sub.IL is the maximum incident angle of the exposure light IL incident on the reticle R, and .theta..sub.EL is the maximum incident angle of the illumination light EL for focal position detection, which is incident from the lower surface of the pattern plate 15.
In general, when the numerical aperture NA is increased, the resolving power is increased, but the depth of focus is decreased. On the other hand, when the .sigma. value is decreased, the coherency of the exposure light IL or the illumination light EL is improved. For this reason, when the .sigma. value is decreased, the edge of a pattern is emphasized. When the .sigma. value is large, the edge of the pattern is blurred, but the resolving power for a finer pattern can be increased. Therefore, the imaging characteristics of the pattern are determined by almost the numerical aperture NA and the .sigma. value. When the .sigma. value is changed, the illuminance distribution of the pupil plane Ep of the projection optical system PL is changed.
Referring back to FIG. 30, when the illumination beam of the exposure light for the reticle R is IL1, assume that .sigma..sub.EL as the .sigma. value of the focal position detection system using the pattern plate 15 is equal to .sigma..sub.IL as the .sigma. value of the exposure light IL1. In this state, when the illumination beam of the exposure light is changed to IL2 in accordance with the type of reticle R, .sigma..sub.EL of the illumination light EL is no longer equal to .sigma..sub.IL of the exposure light IL2.
In recent years, a method using a phase shifting reticle is assumed to increase the resolving power. In this case, a better effect can be obtained when the .sigma. value of the illumination system on the exposure light IL side is set small (e.g., .sigma..sub.IL =0.3). The .sigma. value of the illumination system on the exposure light side is about 0.5 to 0.6 for the normal reticle. An exposure apparatus for selectively using the above two .sigma. values is proposed. Since a combination of the numerical aperture NA of the projection optical system PL and the .sigma..sub.IL as the .sigma. value of the illumination system on the exposure light IL side must be optimized to expose a micropattern, an exposure apparatus capable of varying the numerical aperture NA and the .sigma. value on the exposure light IL side is also proposed. In this case, the .sigma..sub.EL value as the .sigma. value of the illumination optical system for focal position detection becomes different from the .sigma..sub.IL value as the .sigma. value of the illumination system for actually performing exposure. When the .sigma. values are different, the focal positions are slightly different in accordance with the aberration distributions in the projection optical systems because the intensity distributions are different in the projection optical systems.
Similarly, when the line width of the aperture pattern of the pattern plate 15 for focus detection is different from that obtained by multiplying the line width of the reticle R subjected to actual exposure with a magnification 1/m of the projection optical system PL, the focal position is changed because the intensity distribution of the diffracted light components within the projection optical system varies. This amount is a maximum of about 0.1 to 0.2 .mu.m, but is not small for recent finer micropatterns.